Put the whole potatoes in cold water without peeling them and bring to a boil. Then drain them and use a potato ricer to mash them while still warm.
While still warm, gradually add flour to the mashed potatoes and mix until obtaining the shape of a ball.
Divide the dough into several pieces, form them into rolls and cut pieces of dough in the desired gnocchi size. Roll each piece on the back of a fork, pressing lightly.
Place the gnocchi on a floured cutting board and set aside until needed.
Now heat butter, flour and lemon zest over medium heat. When the butter has melted, add the lukewarm milk and whisk in quickly to prevent lumps from forming.
Continue cooking over low heat until the desired consistency is reached, adding one of the two grated truffles.
Then bring a pot of salted water to the boil and drop in the gnocchi. As soon as they rise to the surface, remove them with a skimmer.
Pour the milk and truffle cream into the plates, place the gnocchi on top and sprinkle with the remaining grated truffle. Cover with the remaining sauce and garnish with black pepper according to taste.